The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to a method and system for nesting gradient pulses to allow increased gradient slew rates and reduced peripheral nerve stimulation. It will be appreciated, however, that the-invention is also amenable to other like applications.
Magnetic resonance imaging is a diagnostic imaging modality that does not rely on ionizing radiation. Instead, it uses strong (ideally) static magnetic fields, radio-frequency (RF) pulses of energy and magnetic field gradient waveforms. More specifically, MR imaging is a non-invasive procedure that uses nuclear magnetization and radio waves for producing internal pictures of a subject. Three-dimensional diagnostic image data is acquired for respective “slices” of an area of the subject under investigation. These slices of data typically provide structural detail having a resolution of one (1) millimeter or better.
When utilizing NMR to produce images, a technique is employed to obtain NMR signals from specific locations in the subject. Typically, the region that is to be imaged (region of interest) is scanned by a sequence of NMR measurement cycles, which vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well-known reconstruction techniques. To perform such a scan, it is, of course, necessary to elicit NMR signals from specific locations in the subject. This is accomplished by employing gradient magnetic fields denoted (GSLICE, GPHASE ENCODE, and GREADOUT) which have the same direction as the polarizing field B0, but which are configured as needed to select the slice, phase encode and readout to facilitate the imaging. By controlling the strength of these gradients during each NMR cycle, the spatial distribution of spin excitation can be controlled and the location of the resulting NMR signals can be identified.
The magnetic field gradient subsystem of an MRI system is perhaps the most critical subsystem in defining the utility of a scanner. A more powerful gradient subsystem, in general, yields greater applications capability. Gradient field performance is dependent on two factors: maximum gradient amplitude and gradient pulse slew-rate. Gradient amplitude is determined by the current that the gradient amplifiers produce in the gradient coils, and gradient slew rate is the rate at which the gradient amplifiers can change the gradient amplitude.
In many circumstances, the only factor of importance in the generation of a gradient field pulse is the integral of gradient amplitude over the duration of the gradient pulse (i.e. the gradient pulse area). This is true, for example, with slice-select refocusing, phase-encoding, velocity or flow compensation, spoiling, rewinding and readout defocusing gradient pulses. Since the shortest duration gradient pulse of a given area provides the greatest flexibility in selecting pulse sequence echo time (TE) and pulse sequence repetition time (TR), it is highly desirable for the MRI system to produce these gradient pulses with the minimum pulse duration possible given the prescribed pulse area.
For a given gradient subsystem, the minimum pulse duration is obtained with a triangular pulse. In such a case, a gradient amplifier is changing the gradient amplitude rapidly until the amplitude reaches a predetermined value. The gradient amplifier then returns the amplitude to zero at a rapid rate. The maximum rate that the gradient amplifier can change the gradient amplitude is established by the slew rater of the gradient amplifier given in units of Gauss per centimeter per millisecond (G/cm/ms). Ideally, a triangular pulse is the shortest possible gradient pulse for a given gradient amplifier since the gradient amplifier is changing the gradient amplitude at the maximum rate, established/limited by design. A real gradient amplifier, however, has a limit to the amplitude of gradient pulses it can produce. Consequently, when the gradient amplifier must formulate a gradient pulse having an area greater than that of a triangular pulse with the maximum allowable amplitude, the gradient pulse providing optimal duration becomes a trapezoid. As with the triangular pulse the gradient amplitude rises at a rapid rate until the maximum gradient, Gmax, is reached. The gradient amplifier then provides constant gradient amplitude for a period PW, followed by a rapid return to zero amplitude. For the minimum duration gradient pulse, the constant amplitude is applied at the design limit of the gradient amplifier.
The desired area of a gradient pulse is typically determined by scan parameters such as the field-of-view and slice thickness, which are input by the operator just prior to a scan. Once the desired area is known, then the MRI system uses the slew rate and maximum amplitude of the gradient amplifier to determine the timing and amplitude of the gradient pulse. If the slew rate and maximum amplitude of the gradient system are sufficiently high, a triangular gradient waveform, will be produced. Otherwise, a longer trapezoidal gradient waveform must be produced with a consequent lengthening of the minimum possible pulse sequence echo time TE and pulse sequence repetition time (TR).
While one could increase the power of the gradient amplifiers such that the maximum prescribed gradient area can be produced with a triangular gradient pulse regardless of its size, this may not solve the problem, and may present others. Namely, due to physiological effects on the patient, because constraints are placed on maximum gradient switching speed (slew rate) for the gradient fields allowed. Time-varying magnetic fields induce currents in conductive materials and rapidly changing magnetic field gradients can induce currents in a patient being imaged. Under some circumstances, these induced currents can stimulate nerves. Therefore, every MRI pulse sequence employed for human patients must conform to one or more magnetic field rate of change limitations in accordance with FDA regulations. Thus, it is now evident, that even though the gradient amplifiers can deliver a desired triangular gradient pulse of prescribed area to meet the desired requirements, physiological limits may preclude its use. Current MRI systems, therefore, assume the worst possible circumstances and limit the gradient slew rate accordingly.
Most physiological limits placed on the gradient field rate of change are not a single fixed value. Instead, the limit changes as a function of the “transition time” (i.e. the time interval over which the change in gradient field occurs). The reason for allowing higher rates of change (i.e. dB/dt) as the transition time decreases is related to the fact that the electrical sensitivity of neurons decrease with increasing frequency. J. P. Reilly of the Johns Hopkins University Applied Physics Lab has modeled the response of nerve cells and produced an equation predicting the dB/dt threshold for peripheral (PNST) and cardiac nerve stimulation as a function of dB/dt and pulse duration. It should be noted that cardiac stimulation occurs at dB/dt levels about 10 times that of PNST, therefore, a wide margin of safety is realized. The Reilly PNST equation, known as the “Reilly Curve,” is the basis for the FDA physiologic limits on dB/dt.
All gradient coil designs intended for human use will have a physiologic limit given by the Reilly equation. The slew rate, which gives the limit, however, will depend on the effective length of the coil. The physiologic slew rate limit is determined by dividing the Reilly limit by the effective length of the gradient coil, L. Note that, the effective length L is not necessarily the true length of the coil. Rather, the effective length is the distance between the iso-center of the gradient coil and the point within the coil that exhibits the maximum dB/dt exposure for the patient (or operator). This distance should be corrected for deviations from non-linearity. For a cylindrical gradient coil, this distance is roughly equal to the distance from the iso-center of the gradient coil to the location of maximum field variation caused by the coil in the Z direction. In the radial dimensions, however, this distance is equal to the patient-bore radius because the location of maximum field variation is within the walls of the MR system.
Most NMR scans currently used to produce medical images require many minutes to acquire the necessary data. The reduction of this scan time is an important consideration, since reduced scan time increases patient throughput, improves patient comfort, and improves image quality. Thus, it can be seen that there is a need to provide minimum duration gradient pulses that do not operate beyond the physiological limits.